After attaching the databases to the new database server, and before mounting the content database in your new farm, be sure to run the Test-SPContentDatabase powershell command to ensure that there are no pre-upgrade issues or dependencies that need to be resolved that could otherwise lead to a failed upgrade. While the pre-upgrade check is helpful, I ultimately find the Test-SPContentDatabase much more relevant since it is more complete and also looks for any possible missing solutions that were still installed on the source farm.